The invention relates to a generator of meter- or decimeter-long electromagnetic waves, consisting of a resonant structure coupled to a tubular beam of electrons in helical orbits.
This generator is based on an interaction between a tubular electron beam, on the one hand, which is given a cyclotronic movement by a static magnetic field, and on the other hand an electromagnetic field with azimuthal distribution set up in a resonant structure, at a frequency close to the cyclotronic frequency of the electrons. Such an interaction is already known per se, but only when the electron beam is coupled to the electromagnetic field of a cylindrical or spherical resonant cavity. It is described, for example, in an article by R. Le Gardeur published in the minutes of the 5th International Congress on Tubes for Hyperfrequencies, Paris, 14-18 September 1964, pages 522 to 526.
The resonant mode used in the interaction described in this article is of the type TE.sub.011 (for transverse electric) in cylindrical geometry. It is identified by three indices m, n, p which characterize the distribution of the field as a function of the polar angle .PHI., the radius r, and the ordinate z counted along the axis, respectively. The electric field corresponding to it has only one tangential component E.sub..PHI., while the radial and axial components are zero and this tangential component is independent of .PHI., and undergoes only one alteration along a radius and one alternation along the axis. This mode is termed "azimuthal" or else "magnetic dipole".
This component E.sub..PHI. of the electric field is given quantitatively by a term which is found in all the specialist works dealing with the theory of volumes resonating at hyperfrequencies (and particularly in the work "Microwave Electronics" by J. C. SLATER); EQU E.sub.101 =A.multidot.J.sub.1 (x'.sub.01 r/R) sin .pi.z/L.multidot. sin wt
wherein:
.PHI., r and z are the cylindrical co-ordinates, PA1 R is the radius of the cavity and L is its length, PA1 A is a constant, PA1 J.sub.1 is the primary Bessel function and x'.sub.01 is its primary root, PA1 w is the resonance pulsation of the mode.
Near the axis, where r is small compared with R, the Bessel function J.sub.1 (x'.sub.01 r/R) is equivalent to x'.sub.01 r/2R, so that the tangential component of the field is expressed by the approximate equation: EQU E.sub..PHI. =(Ax'.sub.01 /2R)r sin (.pi.z/L) sin wt
which shows that, when z is fixed, the field increases with r in the vicinity of the axis.
The resonance frequency of a cavity or the associated wavelength, which comes down to the same thing, naturally depends on the dimensions of the cavity. For the mode TE.sub.011, these parameters satisfy the equation: ##EQU1## where D is the diameter of the cavity, L is its length and .lambda. is the wavelength.
FIG. 1 shows the variation of D.lambda. as a function of L/D, taken as the variable. It appears that the diameter D is always of the order of several wavelengths and, in any case, is greater than .sqroot.1.49, namely 1.22 times the wavelength.
When the range of operation of the electronic tube is within the range of centimeter waves, the cavity therefore has a diameter of the order of 5 to 10 cm, which does not present any special problems. However, with wavelengths of 30 cm or more (i.e. a frequency of 1000 MHz), the diameter of the cavity is already at least 36 cm, and at 3 m (150 MHz) the diameter assumes a value of 360 cm, which is prohibitive for most purposes.
The length L naturally follows similar variations. Therefore, it is out of the question to use such cavities for generating waves which are tens of centimeters or meters long, with the result that the generators of the type described in the abovementioned publication are ill suited for the production of waves measuring several meters or tens of centimeters in length.